Reactive gliosis at the sites in which potassium clearance was measured was quantitated histologically and correlated with potassium clearance rates. A small, significant correlation was observed, with dense gliosis associated with slowed potassium clearance, although the baseline potassium level appeared no different in actively epileptogenic areas or gliotic areas compared with normal areas. The results suggest a negative correlation between reactive gliosis and rapidity of extracellular potassium clearance.